Reduction of transformer noise due to magnetostrictive effects



Jan. 5, 1960 P. w. NEURATH 2,920,296

REDUCTION OF' TRANSF'RMER NOISE DUE lTO MAGNETOSTRICTIVE EFFECTS Filed Jan. 7, 1955 United States Patent O REDUCTION OF TRANSFORMER NOISE DUE T MAGNETOSTRICTIVE EFFECTS Peter W. Neurath, Lenox, Mass., assignor to General Electric Company, a corporation of New York Application January 7, 1955, Serial No. 480,387

9 Claims. (Cl. 336-100) The present invention relates to induction apparatus, and to the reduction of noise resulting from the operation thereof. More particularly, the invention relates to the treatment of magnetic material used in induction apparatus such as transformers to reduce their noise of operation due to magnetostrictive effects in the magnetic material.

It is well known that many materials when magnetized or demagnetized undergo changes in dimension, an effect called magnetostriction, which is defined as or the change in length per unit length in the direction of magnetization under the magnetizing force applied. The magnetostrictive effect is an important factor in the manufacture of electrical induction apparatus, and particularly of power transformers, since it is a basic cause of the noise emanating from the transformers during normal operation.

The improvements in magnetic materials with respect to core loss and permeability which have been achieved in the past have made it feasible to operate transformers at higher induction than has been utilized heretofore. The magnitude of the magnetostrictive effect, however, and the noise resulting therefrom increase with the induction, and for this reason in many cases it has become necessary to design transformers to operate at lower inductions than that at which they would take full advantage of the otherwise excellent magnetic properties of the core material.

Various attempts have been made in the past to reduce such noise. One such attempt has involved abrading the surface of magnetic sheets in order to decrease the magnetostrictive effect, but this method was unsatisfactory in that abrasion was found to increase the watt losses of the magnetic material. Another method involving oxidation of the magnetic steel also resulted in high watt losses in the treated material. It has also been attempted to reduce magnetostrictive noise by employing core steel of increased silicon content, for example about 6%, but this has proved impractical due to the extreme brittleness of such a core material.

It is, therefore, an object of the present invention to reduce noise in induction apparatus such as transformers caused by magnetostrictive effects, while avoiding the above-mentioned disadvantages.

It is another object of the invention to provide magnetic materials having reduced magnetostrictive effects, and methods of producing such materials, so as to avoid noise caused thereby under induction conditions.

It is still another object of the invention to provide magnetic materials and a magnetic core assembly made therefrom Vfor induction apparatus such as transformers wherein the magnetic materials are pre-tensioned to counteract compressive stresses encountered in the core assembly to ,reduce vmagne,tostriction in the magnetic watt losses),

materials to a minimum, thereby reducing the noise produced by the core assembly upon energization of the induction apparatus.

As a result of careful investigation on the phenomena of magnetostriction, particularly as applied to 3% oriented silicon steel sheet material, it has been found in accordance with the invention that the major cause of the present high noise level due to magnetostriction is the critical dependence of magnetostriction on stresses. The magnetostrictive effects in magnetic steel can be increased six-fold by very slight compression of the sample, e.g., of about 300 pounds per square inch, which can be produced by bending a sample very slightly. Since magnetic sheet laminations are always bent appreciably when assembled into a transformer core under the usual manufacturing methods, the transformers turn out to be noisy.

It has been found, for example, that 3% Si-Fe transformer core sheet material of .012 inch and 0.14 inch thickness is quite sensitive to compression in the effects produced on magnetostriction. Compressive stress of the order of 1000 p.s.i. raises the magnetostriction from a strain free level of less than 1 l0-6 (at 15,500 gauss) to a strained level of up to ten times that amount. Such compressive stress is always encountered as a consequence of the assembly of laminations that are not absolutely flat, which is the case of the usual type of magnetic steel laminations used for transformer cores.

I have discovered that such adverse effects of compressive stresses on magnetostriction can be largely overcome by prestressing the magnetic sheet material in tension to a tensile stress value which is at least as high as the compressivestresses subsequently imposed during the assembly process. If, for example, an unstressed material of low magnetostriction is subjected to such a pretensioning stress and as a result has a state of tension after assembly, a low magnetostriction core is obtained.

The prestressing of the magnetic steel can be done 1n various ways. However, it is desirable that the method used to be economical, that it have no adverse effects on the quality of the finished material (e.g., permeability or and that it be compatible with insulation, space factor and oil contamination requirements.

A practical method of providing suitable pretensioning of magnetic steel which has been found effective is by coating the surfaces of the magnetic materials with a varnish or similar material and then subjecting the coated magnetic sheet to an annealing treatment. The coating is preferably applied on each side of the metallic sheet and is of suitable thickness to produce the desired degree of tension on the final magnetic strip before assembly.

This method of producing sufficient tensile stresses in magnetic sheet materials to counteract compressive stresses produced in subsequent assembly of the sheet materials is a significant feature of the invention in providing a practical means of pre-tensioning. However, optimum results in reducing noise in the finally assembled magnetic structure are produced in accordance with the invention by a combination of steps, including the above coating process, which assures low magnetostriction of the finally assembled cores. To this end, it is important to start with magnetic sheet material which has been properly annealed to develop the magnetic qualities of the material while giving it low magnetostriction. The thus annealed material is then subjected to an appropriate coating process as described herein to confer thereon a tensile stress of suitable amount, depending on the magnitude of the bending (compressive) stresses encountered in assembly. In conjunction with the above quality annealing and coating steps, additional measures may be taken to control the amount of bending stresses produced on the sheet material by the nal assembly, as described more fully herein.

The invention `will be better understood from the followiiig description and` in view of the accompanying drawing in` which:

Fig. 1 is a view in elevation of a transformer core assemblywhich may embody the present invention;

Fig. 2 is an enlarged fragmentaryy View in perspective of a lower corner of the transformer core assembly shown in Fig. 1'; and

Fig. 3 graphically illustrates ho'w magnetostriction in a steel sample may be reduced in accordance with the invention.

Referring now to the drawing, and particularly to Fig. 1, there is shown a transformerl core assembly l having a plurality of vertical legs 2, andlower. andupper yoke members 3, 3 across the bottom and top of the legs 2,

the legs 2r and yoke members 3, 3' each being comPOSGd of a plurality of superimposed magnetic steel laminationsl 4. As more clearly shown in Fig. 2, bolts `5 passing through legs 2, and clamp members 6, 6"hel'd by bolts 8, serve to keep the laminations in assembly, thebolts 5 as well as clamp members 6', 6' producing, by their pressure, compressive stresses on the laminations in the iinal assembly of the transformer core. Also, it will be noted that spacers 7 are provided between the clampsV 6, 6 and the lamnatio'ns of the lower yoke 3 to keep thelatter in assembled condition, the spacers 7.likewise serving to produce a compressive stress on the` laminations. The compressive stresses'herein referred toas being a primary cause of increased magnestostriction in the final assembly of the core are the compressive stresses along the length of the laminations, that is, in thedirection of the orientation of the magnetic grains and of the llux induced in the magnetic steel.

`Referring now to the graph in Fig. 3, the curve shown is for a particular magnetic sheet sample magnetized to saturation. It shouldk be noted that while the magneto- `stricti0n values shown are for saturation ux density, the curves pro'duced at flux densities of less than saturation, eg., 15,500 gauss, correspond in shape to that shown in the graph, the particular values of magnetostriction varying in accordance with the different flux densities. The curves for other samples with initially lower magnetostriction can be superimposed on the curve shown by simply shifting the origin of the stress scale.

Ity should be mentio'ned in connection with the graph that since the actual point of zero internal stressy of the magnetic sheet material cannot be accurately determined, the zero stress point has been arbitrarily selected as shown for a better understanding of the principle involved in the invention. It is to be understood, therefore, that the tensile stress indicated in the graph is due to the initial anneal actually represents relief of internal compressive stress and is not strictly the net tensile stress exerted on the material. A properly initially annealed sample will start at a.l point G on the curve lower than 1x10-6 magnetostriction, and any tensile stress put on by the coating will displace the stress point on the magnetostriction curve to some point T. Subsequent compression, due to bendingin assembly, will increase the magnetostriction and will move back up the curve to the nal condition, eg., to point F as shown in Fig. 3. It is this final point that must be controlled in order to keep the noise in the transformer to a minimum. It is clear that this linal point F depends on the following three factors: (1) the starting magnestostriction point (G) of a properly annealed sam- (.2). the amount oftension produced by the coating, and, (3)`the amount of subsequent bending due to the assembly conditions.

It should be noted that thev curve shows magnestostriction in relation to total stress in themagnetic sample, where the total stress is the sum of the externally applied stress and .the effective `internal stress. The latter depends on the prior treatment ofthe sample before coating, and the point G indicates on the curve approximately the amount of internal stress of a sample annealed at approximately 1175 C. The increased tension shown on the graph by the tension arrow is produced in the particular instance shown by a coating of Sterling varnish about 0.3 mil thick on the sample. row indicates an amount of bending stress which is typically produced in the transformer core assembly, vand it will be seen that this amount ofv subsequent compressive stress has been more than adequately counteracted by the combined effects of proper annealing and external pre-tensioning of the sample due to the coating. Improper annealing and/or an insufiicient amount o'f pretensioning on the sample .will cause the ultimate compressive stress to bring the nal point to the more nearly vertical portion of the curve, the sample thus being in a condition where very` slight increasesin bending stress will result in considerably greater'V magnetostfiction'and, consequently, vgreater Anoise from theV transformer core assembly The temperature at which the initial anneal of the magnetic materialk is carried out 'has' :been foundl to greatly inuence the resulting magnetostric'tion'. After an anneal at 1000" C. such as has been commonly used heretofore, themagnetostriction values may range from 3 to20 1.0"6 at saturation flux density with an average value approximately as indicated by point A on the'graph. If a higher temperature o'f annealing is used, eg., 1100 C., a much smaller range of magnetostriction values will be found, with an average of approximately the amount indicated by point B on the curve. The annealing temperature which has been found to give the lowest /magnetolstri'ction while still producing optimum magnetic properties in the steel is in the range of 1100 C. and` up, with 117:5? C. preferred, and the state of magneto'striction ofV samples thus annealed is indicated by pointG on the graph;

Coatings which haveL been foundparticularly suitable in accordancewith the invention for producingtension on the samples are generallyfof a phosphate composition, and more specifically the coatings may be formed from a mixture of at least one refractory oxide and ,phosphoric acidy in water. Thelfollowing are. two'particular coating compositions found esptirciallyv suitable, the proportions being given in parts by weight-z y Barts MgO (USP heavy) 3,;9 HgPO., 3.8.2 H2O 57.9

. Sterliugyamish Parts S102 f..... .V.-V f- 55 F6203 1, .l. A1203 13.17 P205 29.89 H2O 50.1.7

Other coating compositions found suitable. include aluminuml phosphate, Al2O3i3P2O`5,I with thev ratio of aluminum oxide yto Yphosphorus pentoxide, varying from 1:3 to 2:3. Thefollowing compositionslin.aqueous solution are illustrative of this type Ofivcoating material, the proportions being in parts byr weight: I

The compression ar-V The amount of water in the above'fcompo'sitions may .be varied to give different; 'consistenci'e's of theinixturc and" to produce different Vthickii'esses'fin` the ral coating. AThe coating may be applied in` any'y suitable' manner, as by spraying, dipping, brushing, or thf like; After"'tl`1` coating is applied, the coated sheet material is preferably heated to about 790 to firmly bind the coating to the surface of the metal. If residual internal stresses are present in the coated metal, due, for example, to cutting of the metal into laminae or to too rapid cooling after the initial quality anneal, or for any other reason, this anneal at 790 serves as a stress relief anneal to eliminate such internal stresses. For such purposes, the anneal is carried out at 790 C. in H2 or N2 gas or in air for about 8 hours as a batch anneal, or as a continuous anneal at the same temperature for a few minutes.

The following table illustrates the approximate amount of tension which may be produced in magnetic steel strips by Sterling varnish coatings of different thicknesses:

Tension on Thickness of Sterling Varnish Coating, mils Sample,

sion of between 300 and 1400 p.s.i. Sterling varnish is preferable for the greater thicknesses, since the C-l0 composition is not easily applied in thicknesses over 0.3 mil. It has been found that thicknesses of the coating over 1.0 mil do not produce added benefits in terms of increased tension or reduction in magnetostriction. Furthermore, too thick a coating is undesirable in that it takes up an excessive amount of space in the transformer core. Also, too great a thickness of the coating may cause breaking olf of the coating due to concentration of compression at the interface between the coating and the metal surfaces, since the coating itself is subjected to longitudinal compression due to the tension it produces on the metal surface. Smaller thicknesses than `the above range are not desirable since they do not produce the necessary amount of tension.

In order to test the magnetostrictive properties of the samples treated in accordance with the invention, the measurements were made with the samples curved to a predetermined radius of curvature. It was found that bending the samples to an 8 foot radius of curvature produced a compressive stress approximately equivalent to the amount of stress usually encountered in the assembly of the magnetic strips into a transformer core.

In a series of comparative magnetostrictive tests made on coated and uncoated Epstein strip samples (25 cm. X 3 cm. x .014 inch) annealed in H2 at l000 C. where the magnetostriction values were obtained with the samples bent to 8 and 16 foot radii of curvature, the following results were obtained:

From the above table it is evident that the magnetostriction of the coated samples was substantially reduced below that of the uncoated samples. Comparison of the values obtained as between the samples of 8 and 16 foot radii of curvature also shows that the greater'the corn- 6 pressive stress to which the samples are ultimately subjected, the more marked is the reduction of magnetostriction produced by the present coating process.

It will be apparent from the foregoing that the smaller the radius of curvature, the greater the compressive stress, and hence the greater the magnetostriction, in the magnetic strip. Since, as pointed out above, thicker coatings produce greater tensile stress for counteracting greater compressive stress, it is evident that in accordance with the invention the thickness of the coating applied is inversely proportional to the radius of curvature of the strip.

As indicated previously, the final compressive stress to which the treated magnetic strips are subjected may be controlled by additional measures and by thus restricting the compressive stresses, thinner coatings may be used to produce the requisite tension for counteracting the ultimate bending stresses. One method which may be used in conjunction with the coating process is the stress relief anneal already described. Another supplementary method is the process of heat flattening, already known by the art, which may be associated with the coating process and which consists generally in applying external tension to the strip while it is being heated to a suitable temperature. By this method, the curvature of the strips may be reduced, so that in final assembly less bending stress will be encountered. A further measure is the socalled in phase annealing in which care is taken to nest the laminations during the stress relief anneal in the same order that they will be assembled in the nal core structure, so that the laminations, if they have any remaining curvature, will be curved in substantially the same direction in the final assembly and thus minimize compression due to bending stresses.

Any or all of the above supplementary measures may be used to control the bending stresses encountered. In this connection it might be noted that commercially produced annealed steel sheets are commonly classified in terms of radii of curvature to indicate the relative amount of warping of the sheet. The present linvention is of particular value in this regard, since it is only a simple matter to determine how much tension is necessary to be produced by the coatings applied (and hence the necessary thickness of the coating required) to overcome the amount of bending stresses which would be expected by final assembly of a magnetic steel strip warped to the specified radius of curvature.

The following procedure is a particular process which may be carried out in accordance with the present invention to provide low magnetostriction in transformer core laminations subjected to the usual assembly stresses. The magnetic steel material, which may be in coiled form and having an MgO separator coating on its surfaces, is subjected to a quality anneal at 1175 C. in dry hydrogen. After cooling, the steel surface is cleaned, eg., by light pickling and/or wire brushing. The steel is then coated with Sterling varnish to a thickness of about 0.3 mil per side and is then heat flattened to take out coil set and ripple. The material is then cut and batch annealed for 8 hours at 790 C. in N2.

These steps insure an adequate quality anneal, low ripple, and a firmly adherent coating with good insulating qualities and which produces an average tension of at least 400 p.s.i. in the steel.

While 3% Si-Fe steel has been mainly referred to in describing the invention, primarily because this material is most commonly used at present as transformer core steel and appears to be most sensitive to compressive stresses Ain terms of increased magnetostriction, other silicon-steel compositions or even pure iron can also be treated in accordance with the invention to produce lower magnetostriction. Thus, similar treatments of steel having up to 6% silicon content where the material is grain oriented is contemplated in accordance with the invention.

The present invention makes it possible not only to '7 reduce the magnetostriction of magnetic steel material in itself, but also, and more importantly, to provide llow magnetostriction in the ,core structure as finally assembled, and it is the concept of the critical role played by the compressive stresses in assemblyv in` increasing magnetostction `and the resulting noise which, so far as is known, has not been previously recognized in the art and which forms the basis of the `present invention. This problem now having been recognized, the present invention aproe vides effective, practical and economical measures to counteract and control such compressive stresses, and while producing these desirable results, it does not result in an increase of watt losses of the treated material, as is the lcase in prior artmethods of reducing kmagnetostriction which rely'solely onoxidation and abrading procedures.

Although the most practical method of 'producing tension inthe steel strips isby the application of particular coatings `as described above, other methods of applying tension could also ,be used, such as the following: application of a fused` Pyrex, Vycor, or quartz coating athigh temperature to produce tension upon cooling; a similar application of special low expansion glasses; plating, `such as with Cr2O3, to produce tension; application of mechanical stress (tension) by the clamping structure `of the core; and use of suitable porcelain enamels, ceramic coatings, or solution ceramics. Another possible method is the application at sub-zero temperatures of a suitable organic, inorganic or metallic coating such that on heating the strips up to ambient temperature a tension :is produced therein.

Accordingly, the appended -claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A magnetic core assembly for induction apparatus comprising annealed grainoriented magnetic sheet material; means holding said sheet material in assembly While exerting compressive stress thereon in the direction of grain-orientation of said sheet material; and means producing on said sheet material at least an amount of tension to fully counteract the compressive stress exerted by said holding means on said sheet material, whereby the magnetostriction of the sheet material and the consequent noise produced when the induction apparatus is energized are reduced.

2. A magnetic core assembly for induction apparatus comprising annealed grain-oriented magnetic sheet material; means holding said sheet material inkassembly while exerting compressive stress thereon in the direct-ion of grain-orientation of said sheet material; and'means cornprising a coating of a thickness of .2-l.0 mil on the surfaces of said sheet material for producing on said sheet material at least an amount of tension to fully counteract the compress-ive stress exerted by said holding means on said sheet material, whereby the magnetostriction of the sheet material and the consequent noise produced when the induction apparatus is energized are reduced.

3. A magnetic core assembly for transformers comprising a plurality of laminae ofV annealed magnetic strip material formed of a silicon-iron alloy containing up to 6% silicon; clamping means holding said laminae in assembly while exerting compressive stress thereon along their length; and a coating formed from a mixture of a least one refractory oxide and phosphoric acid in water and being of from .2l.0 mil thickness on the surfaces of said laminae for producing in said magnetic strip material an amount of tension greater than required to fully counteract the compressive stress exerted by said clamping means on said strip material so as to leave a residual tension on the assembled strip material, whereby the magne-y tostriction of the strip material and the consequent noise produced when the transformer is energized are reduced.

4. A magnetic core assembly for transformers comprising a plurality of strips of annealed magnetic material which Aare grain-oriented along their length and which in unassembled form .have portions respectively bent to predetermined radii of curvature about axes transverse said tstrips; means :holding said strips Yin assembly superimposed on each other in substantially flat condition while fexertingcompressive stress thereon along their length; and coating means of a thickness of .2-1 .0 mil on at least said bent portions of said strips for producing Vtensiontherein counteracting the compressive stress exerted vvby `said rholding means, the thickness of said coating means on said bent portions being inversely proportional 'to their respective radii of curvature, whereby vthe magnetostriction ofthe strips andthe, consequent noise produced when thetransformer is energized are reduced.

5. The method of reducing magnetostriction in mag netic sheet material adapted for-use yas core .structure in induction apparatus wherein said magnetic sheet material Vis subjected to compressive stress in assembly in the Vinduction apparatus, which A'method comprises annealing the magnetic sheet ,material for developing the magnetic properties and reducing the magnetostriction therein, and forming a coating of a thickness of `.2-l.0 mil on the magnetic sheet material and heating thecoated material to rmly bind the coating to the sheet material ,for producing at least an amount of tension on the thus annealed low magnetostriction magnetic sheet material to fully counteract the compressive stress encountered by said sheet material in assembly, whereby the magnetostriction of the sheet material and the consequent noise produced by the coreY assembly 'when the induction apparatus is energized are reduced.

, 6.v The-method of reducing magnetostriction in magnetic stripmaterial adapted for use as core 'structure in inductionV apparatus wherein said magnetic strip material is subjected to compressive stress lin assembly in the induction apparatus, which method comprises 4annealing the magnetic strip Vmaterial for Ldeveloping the magnetic properties and reducing the magnetostriction therein to less than 4X l06 at saturation llux density, and forming a coating .of a thickness of .2-l.'0 mil on the magneticstrip material and heating the coated material to rmly bind .the coating to the strip material for producing at least an amount of tension on thefthus annealed low magnetostriction magnetic Vstrip material to fully counteract the compressive stress encountered by said strip material in assembly, whereby the magnetostriction of the strip material and the consequent noise produced by the core assembly when the induction apparatus .is energized `are reduced. n

7. The method of reducing magnetostriction in magnetic strip material adapted for use as core structure in induction apparatus lwherein said magnetic Astrip material is subjected to compressive stress in assembly in the in- -duction apparatus, which method comprises annealing the magnetic strip material for developing the magnetic properties and reducing the magnetostriction therein, and forming on the surfaces of the strip lmaterial arcoating comprising a phosphate compound and havingl a thiclo ness of from 0.2-1.0 mil, said coating being vformed from a mixture of a phosphoric compound and at least one refractory' oxide in water, and heating the coated material to firmly bind the coating to the strip material for producing an amount of tension on the thus annealed low magnetostriction magnetic strip material greater than required yto fully counteract the compressive stress encountered by said strip material in assembly so as to leave a residual tension in the assembled strip material, whereby the magnetostriction of the strip material 'and the consequent noise produced by the core assembly when the induction apparatus is energized are'redu'ced.

8. The 'method of reducing magnetostriction inningl netic strip material adaptedfor use as Vcore structure in induction apparatus` wherein said magnetic strip lmaterial is subisse@- ts @agressive stress in assembly is the in:

duction apparatus, which method comprises annealing the magnetic strip material for developing the magnetic properties and reducing the magnetostriction therein, applying on the surfaces of the strip material a coating comprising a mixture of a phosphoric compound and at least one refractory oxide in water, suicient to provide a final coating .2-l.0 mil thick and subjecting the thus treated strip material to a second anneal at about 790 C. to relieve the internal stresses therein as well as to firmly bind the coating to the surfaces of the strip material and thereby produce by said coating at least an amount of tension on the thus treated magnetic strip material to fully counteract the compressive stress encountered by said strip material in assembly, whereby the magnetostriction of the strip material and the consequent noise produced by the core assembly when the induction apparatus is energized are reduced.

9. The method of reducing magnetostriction in silicon steel magnetic strip material adapted for use as core structure in induction apparatus wherein said magnetic strip material is subjected to compressive stress in final assembly in the induction apparatus, which method comprises annealing the magnetic strip material for developing the magnetic properties and reducing the magnetostriction therein, applying on the surfaces of the strip material a coating comprising a mixture of a phosphoric compound and at least one refractory oxide in water sufficient to provide a nal coating .2-1.0 mil thick, subjecting the thus treated magnetic material in the form of superimposed strips to a second anneal at about 790 C. for about 8 hours to relieve the internal stresses therein as well as to irmly bind the coating to the surfaces of the strips and thereby produce by said coating at least an amount of tension on the thus treated magnetic strips to fully counteract the compressive stress encountered by said strips in final assembly, and arranging the strips in final assembly in the same order and position in which they were arranged during said second anneal so as to reduce the bending stresses exerted during assembly,

lwhereby the magnetostriction of the strips and the consequent noise produced by the core assembly when the induction apparatus is energized are reduced.

References Cited in the le of this patent UNITED STATES PATENTS 1,857,215 Ruder May l0, 1932 2,234,968 Hayes et al Mar. 18, 1941 2,319,775 Mittermaier May 18, 1943 2,394,047 Elsey et al Feb. 5, 1946 2,467,868 Somerville Apr. 19, 1949 2,484,242 Nagel et al Oct. 1l, 1949 2,492,095 Gifford Dec. 20, 1949 2,494,349 Mittermaier Jan. 10, 1950 2,501,349 Nagel et al Mar. 21, 1950 2,501,846 Gitlord Mar. 28, 1950 

1. A MAGNETIC CORE ASSEMBLY FOR INDUCTION APPARATUS COMPRISING ANNEALED GRAIN-ORIENTED MAGNETIC SHEET MATERIAL, MEANS HOLDING SAID SHEET MATERIAL IN ASSEMBLY WHILE EXERTING COMPRESSIVE STRESS THEREON IN THE DIRECTION OF GRAIN-ORIENTATION OF SAID SHEET MATERIAL, AND MEANS PRODUCING ON SAID SHEET MATERIAL AT LEAST AN AMOUNT OF TENSION TO FULLY COUNTERACT THE COMPRESSIVE STRESS EXERTED BY SAID HOLDING MEANS ON SAID SHEET MATERIAL, WHEREBY THE MAGNETOSTRICTION OF THE SHEET MATERIAL AND THE CONSEQUENT NOISE PRODUCED WHEN THE INDUCTION APPARATUS IS ENERGIZED ARE REDUCED. 